The present invention concerns a system for preventing fan-out in a web offset press. "Fan-out" is a condition in which the color printed by the first printing unit is wider across the web than the colors printed by the subsequent printing units.
When the web leaves the feed roll, it is under relatively high longitudinal tension. Thus prior to entering between the blanket cylinders, the web is effectively laterally shortened. As the web enters the blanket cylinders of the first printing unit and the printing occurs, the web expands laterally. Thereafter, the printing of the other colors is on a web that has expanded laterally while the first color was being printed. Since the first printing is on paper that is slightly corrugated, when the paper expands laterally it will have a laterally larger image. This fan-out predominates on the first color, because after the first color is printed, the paper is acclimated to the press and it does not have the slightly corrugated nature that it had as it was fed into the blanket cylinders of the first printing unit.
Certain methods for solving the fan-out problem have been attempted. For example, in one prior art method the printing is accomplished on a proof, and the operator thereafter measures the fan-out and reduces the negative laterally for the first color. The first color plates are then remade, having smaller lateral dimensions in order to compensate for the fan-out. This method is relatively expensive in press made ready photography, press time and paper loss. In addition, it is uncertain that the next roll of paper will fan-out the same amount, and the method may have to be repeated for each roll of paper.
Another prior art method of attempting to correct the fan-out problem is the use of bustle wheels. To this end, a wheel (or wheels) is put under the printed web to reduce the lateral dimension of the web by placing a longitudinal corrugation or wave into the web. The use of bustle wheels requires constant adjustment, may puncture the web and the wheels may pick up ink and redistribute the ink.
Another prior art method of attempting to solve the fan-out problem is to extend the length of the web before it enters the first printing unit. However, this method by which the length of the web lead in the infeed section of the press is increased has not been found to be significantly effective in reducing fan-out.
It is, therefore, an object of the invention to provide a system for controlling fan-out which does not require remaking the first color plates, does not require bustle wheels and does not require an exceptionally long web lead in the infeed section of the press.